Reduction of transport in stellarators by self-shielding

Conference · Tue Dec 31 04:00:00 EST 1996

OSTI ID:489432

Boozer, A H ^[1]

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1. Columbia Univ., New York, NY (United States)

The enhancement of neoclassical over Pfirsch-Schlueter transport at low collisionality is the effect of the parallel viscosity. The parallel viscosity allows the external coils to produce forces within the magnetic surfaces. These forces are balanced along the magnetic field by a poloidal and toroidal variation of the plasma pressure orthogonal to the field, p{sub {perpendicular}}. The oscillatory part of p{sub {perpendicular}} gives a radial force that is balanced by shielding currents. If the Fourier components of the perpendicular pressure and the field strength are P{sub MN} and {epsilon}{sub MN}B{sub o}, then a Fourier component of the field is shielded when the parameter {Delta} = ({mu}{sub o}/B{sub o}{sup 2})(P{sub MN}/{epsilon}{sub MN}) becomes larger than unity. Each Fourier component in the perpendicular pressure also produces an in-surface force, which is balanced by a radial current. This non-ambipolar transport is characterized by a diffusion coefficient D{sub MN} = (M/2) D{sub B} (P{sub MN}/P) {epsilon}{sub MN} with M the poloidal mode number and the Bohm-like diffusion coefficient D{sub B} = (T/eB)/(-dln(n)/dln(r)). In the absence of external forces, such as neutral beams, quasineutrality implies the overall viscosity induced transport must be ambipolar, and {Sigma}P{sub MN}{epsilon}{sub MN} = 0. That is, the parallel flow and radial electric field will adjust themselves to insure ambipolarity and {Sigma}P{sub MN}{epsilon}{sub MN} = 0. Nevertheless, the individual Fourier terms, the p{sub MN}{epsilon}{sub MN}, need not vanish, and a non-zero P{sub MN} generates shielding currents. The heat transport of a species is approximately given by the P{sub MN} of that species. Even if a P{sub MN} vanishes, the individual contributions of the electrons and ions to the P{sub MN} need not vanish.

• DOE Contract Number: FG02-95ER54333
• OSTI ID: 489432
• Report Number(s): CONF-960354--
• Country of Publication: United States
• Language: English

Related Subjects

70 PLASMA PHYSICS AND FUSION TECHNOLOGY
DIFFUSION
MAGNETIC SURFACES
NEOCLASSICAL TRANSPORT THEORY
SELF-SHIELDING
STELLARATORS
VISCOSITY